Many microwave popcorn popping constructions in common commercial use are multi-ply paper bags in which inner and outer paper sheets are laminated to one another, with a microwave interactive construction (sometimes referred to as a microwave susceptor) encapsulated between the paper plies. Popcorn popping bags of this type are described, for example, in U.S. Pat. Nos. 4,904,488; 4,973,810; 4,982,064; 5,044,777; and 5,081,330, the disclosures of which are incorporated herein by reference.
A common feature of such constructions is that they are generally made from flexible paper materials. In this manner, the constructions are sufficiently flexible to open or expand conveniently under steam pressure, when a popcorn charge therein is exposed to microwave energy in a microwave oven. Also the materials are sufficiently flexible to be formed from a sheet into a folded configuration, for example during a continuous bag-construction process.
Many microwave popcorn products include, within the bag, a charge of unpopped popcorn kernels, fat/oil (i.e. grease) and flavor (for example salt). The fat/oil is typically in a form which is not liquefied until at least about 105.degree. F. However, during storage or shipment, especially if the environment becomes relatively hot, the material stored within the bag can become liquefied and leak through the bag construction. Even when relatively high temperatures are not encountered in storage, some leakage can occur if the stored material includes a significant amount of flowable or liquefied oil/fat.
In addition, conventional microwave cooking of popcorn (especially when the popcorn charge includes fat/oil) results in the generation of hot liquid oil or fat. If the construction retaining the popcorn charge is paper, the paper must be sufficiently resistant to staining and to the passage of hot liquid oil/fat therethrough, during the microwave cooking process, to be satisfactory for performance of the product. For example, the oil/fat should not leak from the construction, when the microwave cooking (i.e. popping) is undertaken, sufficiently to generate an undesirable greasy feel or appearance, to the outside of the package.
Greaseproof papers have been developed for utilization in constructions which must, to some extent, resist the passage of oil-like liquids, such as hot liquid oil/fat, therethrough. In general, during construction of a greaseproof paper, the pulp is abraded so that when the greaseproof treatment is cast on it, substantial hydrogen bonding in the cellulose occurs. This process of abrading the pulp is generally referred to as "refining". Typically the more refined the paper is, the more brittle it is. Thus, if a heavy, strongly greaseproof, paper is utilized, a relatively rigid, brittle (nonflexible) construction results.
A general trend, then, is that while a paper system can be made readily greaseproof by abrading, for retaining of oil therein, such a construction will generally be brittle and not of desirable flexibility or strength for ease of assembly, folding, filling, storage and/or use. Also, should a crease or sharp fold (i.e. discontinuity) develop in such material, a leak can readily occur along that resulting discontinuity.
In order to provide some flexibility in the greaseproof paper, modern greaseproof papers involve some refinement of the pulp and some chemical treatment. With less refinement, the resulting paper is less brittle. However, in general such greaseproof papers have not been found to be fully desirable, by themselves, as the construction material for microwave cooking constructions.
A typical, conventional approach to this is for an arrangement of flexible paper in microwaveable systems to have a multi-ply construction, with at least one layer of greaseproof paper bonded to a layer of kraft paper. As a result of such a composite or multi-ply construction, a paper construction material can be readily provided which is both flexible and greaseproof.
While such constructions have been provided for microwave constructions, especially those for retaining microwave popcorn, continued improvement is sought. For example, chemical treatments for rendering refined papers greaseproof and/or grease-resistant typically involve flurochemicals. In some applications, it would be preferred, if possible, to avoid or reduce fluorochemical use, or papers which have been treated with substantial amounts of fluorochemicals.
Further, the laminating adhesives used in the multi-ply arrangements often include polyvinyl acetates. A typical one is Duracet 12, available from Franklin International of Columbus, Ohio 43207. In some applications, it would be preferred, if possible, to reduce the utilization of such adhesives.